Many investors have heard the old saw that “it’s not a stock market, but a market of stocks.” The saying is meant to imply that while we talk a lot about the general direction of a bull or bear market, it’s just as important to acknowledge the qualities of individual stocks.

It’s a mindset that’s worth remembering in 2018, when the S&P 500 Index

which is up more than 70% in the same period, thanks in part to big earnings in October.

This trend isn’t just a recent occurrence. Recent data shared by Longboard Asset Management show that if you back out the top 20% of all stocks since 1989, investors would be sitting on a net return of zero.

Looking at the top five and bottom five stocks in the S&P 500 across different time frames — the top 1% and the bottom 1% — it becomes clear that one big winner can make you a fortune, and just one mistake can leave you in the poorhouse.

Best and worst in the past 30 days

October was a volatile month on Wall Street. But all told, the S&P 500 is down just about 3% in the past 30 days. So why all the fuss? Well, because some stocks had it much worse than others, thanks to specific struggles with their business, like the aforementioned General Electric and its dividend cut. And at the same time, specific news caused a select group of stocks to rocket higher, including IBM

Best and worst in the past 12 months

which has seen its shares roughly triple in the past year, thanks in part to a massive short squeeze in February. But it’s important to note that a short-term surge of roughly 70% in early 2018 was only one piece of the puzzle, and investors who have let this winner run have been well-served.

Similarly, the recent big move downward in GE on a dividend cut is only part of the narrative. This is a stock that has been challenged for a long time.

What’s the trade?

If past is precedent, then investors should take notice of these trends and put their emphasis on picks with near-term momentum that is significantly higher, while dumping big losers ASAP before they get even worse.

Of course, the devil is in the details. Because it’s worth noting that before Fossil’s massive run in 2018, it was one of the worst-performing stocks on Wall Street with minus 70% returns in calendar 2017, owing to weak profits. A dramatic turnaround in sentiment resulted in a dramatic turnaround for this stock.

The stock soared roughly 90% in 2017 but has flopped more than 35% in the past 12 months after a series of poor earnings reports and fears that its China business is in trouble.

It’s important to keep these examples in mind, because no winner will run perpetually higher and there are indeed examples of stocks that come roaring back from the depths. But the basic notion remains that momentum matters, and it’s crucial to pay attention to this fact instead of getting sentimental.

Selling a winner quickly for fear the profits will evaporate can leave a lot of money on the table, and hanging on to a battered stock because you’re hoping for a turnaround often leaves you in even worse shape.